Images captured using digital cameras suffer from brightness and color shading distortions that can compromise the quality of captured photos. Brightness shading, also known as vignetting, is a position dependent decrease in the amount of light transmitted by an optical system causing darkening of an image near the edges. Vignetting, which affects both film and digital cameras, refers to a decrease in the amount of light transmitted by an optical system near the periphery of the lens field-of-view (FOV) causing gradual darkening of an image at the edges. Vignetting can be effectively fixed by calibrating the lens roll off distortion function of the camera.
Color shading is similar in effect and manifests as shift in color near the edges of the sensor. Color shading distortion is specific to digital cameras. The spectral sensitivity of a typical charge-coupled device (CCD) or complementary metal-oxide-semiconductor (CMOS) sensor is higher at the red end of the spectrum of visible light than at the blue end of the spectrum, and also extends considerably into the near infrared (IR) spectrum. The relatively high sensitivity of such sensors to IR light can cause errors in color reproduction. Therefore, in most digital sensors, the IR sensitivity is limited by a thin-film reflective IR filter at the face of the sensor that blocks the infrared wavelength while passing visible light. However, the transmittance of the IR filter shifts to shorter wavelengths as the angle of light incident on the filter increases. Accordingly, longer wavelengths (such as red light) can be blocked more at the edges of the image sensor due to larger incident light ray angles, resulting in a spatially non-uniform color temperature in the image.
Conventionally, color shading artifacts are corrected by per-unit-calibration, which measures color shading profiles of an individual image capture device under a set of illuminants from images of a flat-field scene captured by the image capture device under each illuminant in the set. The inverse of the two-dimensional profile of color shading under each illuminant is stored as a correction table to compensate for color shading artifacts in images captured by the image capture device. When capturing an image, the image capture device first employs a white balance algorithm to detect the scene illuminant from the captured data on the sensor and then selects the corresponding correction table to compensate for color shading distortion.